Conductive polymers are increasingly gaining economic importance, since polymers have advantages over metals with respect to processability, weight and targeted adjustment of properties by chemical modification. Examples of known π-conjugated, conductive polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes). Layers of conductive polymers are employed in diverse industrial uses, e.g. as polymeric counter-electrodes in capacitors or for throughplating of electronic circuit boards. The preparation of conductive polymers is carried out chemically or electrochemically by oxidation from monomeric precursors, such as e.g. optionally substituted thiophenes, pyrroles and anilines and the particular optionally oligomeric derivatives thereof. In particular, chemically oxidative polymerization is widely used, since it is easy to realize industrially in a liquid medium or on diverse substrates.
A particularly important polythiophene which is used industrially is poly(ethylene-3,4-dioxythiophene) (PEDOT or PEDT), which is described, for example, in EP 0 339 340 A2 and is prepared by chemical polymerization of ethylene-3,4-dioxythiophene (EDOT or EDT), and which has very high conductivities in its oxidized form. An overview of numerous poly(alkylene-3,4-dioxythiophene) derivatives, in particular poly(ethylene-3,4-dioxythiophene) derivatives, and their monomer units, syntheses and uses is given by L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik & J. R. Reynolds, Adv. Mater. 12, (2000) p. 481-494.
The dispersions, disclosed for example in EP 0 440 957 A2, of PEDOT with polyanions, such as e.g. polystyrenesulphonic acid (PSS), have acquired particular industrial importance. Transparent, conductive films which have found a large number of uses, e.g. as an antistatic coating or as a hole injection layer in organic light-emitting diodes (OLEDS), as shown in EP 1 227 529 A2, can be produced from these dispersions.
In this context, the polymerization of EDOT is carried out in an aqueous solution of the polyanion, and a polyelectrolyte complex is formed. Cationic polythiophenes which contain polymeric anions as counter-ions for charge compensation are also often called polythiophene/polyanion complexes in the technical field. Due to the polyelectrolyte properties of PEDOT as a polycation and PSS as a polyanion, this complex in this context is not a true solution, but rather a dispersion. The extent to which polymers or parts of the polymers are dissolved or dispersed in this context depends on the weight ratio of the polycation and the polyanion, on the charge density of the polymers, on the salt concentration of the environment and on the nature of the surrounding medium (V. Kabanov, Russian Chemical Reviews 74, 2005, 3-20). The transitions in this context can be fluid. No distinction is therefore made in the following between the terms “dispersed” and “dissolved”. Similarly little distinction is made between “dispersing” and “solution” or between “dispersing agent” and “solvent”. Rather, these terms are used as being equivalent in the following.
There is a great need for being able to structure electrically conductive layers based on conductive polymers, in particular based on complexes of polythiophenes and polyanions, similarly to ITO layers (=indium tin oxide layer), where here and in the following “structuring” is to be understood as meaning any measure which leads to an at least partial reduction, but preferably to a complete elimination of the conductivity, in a part region or in several part regions of the layer of electrically conductive polymers.
One possibility for the production of structured layers based on conductive polymers is to apply these polymers to surfaces in a structured manner via certain printing processes, as is described, for example, in EP-A-1 054 414. However, the disadvantage of this set-up for achieving the object is that the electrically conductive polymers must be converted into a paste, which sometimes causes problems in view of the tendency of conductive polymers to aggregate. Furthermore, during application of electrically conductive polymers via printing pastes there is the disadvantage that the outer region of the drops of liquid is thicker than the inner region and that accordingly on drying of the pastes the coating is thicker in the outer region than in the inner region. The resulting irregularity in the layer thickness often has an adverse effect on the electrical properties of the electrically conductive layer. A further disadvantage of structuring via printing pastes is that this is applied only in those regions in which an electrical conductivity of a substrate surface is desired. The consequence of this is that considerable differences in colour occur on the substrate surface between the regions with and without application of the printing paste, which as a rule, however, are undesirable.
In addition to the use of printing pastes, a further possibility for the production of structured coatings from conductive polymers consists of first producing a uniform, non-structured coating from electrically conductive polymers and only then structuring this, for example by photo-bleaching processes or by the use of etching solutions. Thus, for example, WO-A-2009/122923 and WO-A-2008/041461 describe processes in which layers of electrically conductive polymers are structured by means of cerium ammonium nitrate solutions having an etching action. JP-A-2010-161013 describes a process in which structuring of a layer of a conductive polymer is carried out by using a photoresist and/or a dry film resist in combination with an etching agent solution containing cerium ammonium nitrate, cerium ammonium sulphate or hypochlorite. However, the disadvantage of this set-up is, inter alia, that such etching solutions remove the coating of the electrically conductive polymer to a considerable extent, and because of these changes in the nature of the surface the external appearance of the coating is therefore adversely influenced. In particular, the colour of the coating is impaired decisively by a structuring with etching solutions containing cerium.